Microwave imaging can be used to determine the dielectric permittivity distribution of objects using measurements of a scattered electric field. In a biological context, such imaging of biological tissues having different dielectric permittivities results in different scattering cross sections. The contrast obtained with this imaging is generally greater than that obtained with xray radiography, which is on the order of a few percent. Since the electrical conductivity and dielectric constant of tumors, for example, are an order of magnitude higher than normal tissue over a wide frequency range, typically in the range of 107-1010 Hz, tumors can be readily differentiated from the normal tissue. Moreover, with such imaging irregular physiological changes resulting from the deterioration of health can also be monitored.
Conventional microwave imaging is predominantly based on narrowband technology and records scattered signals at a single frequency, or alternatively, at multiple frequencies recorded at different times. A pulsed microwave imaging system on the other hand, contains a wide range of frequencies. For example, a Gaussian-pulse varies in time according to the following relation:g(t)∝exp(−t2/a2); and  (1)the corresponding frequency range is given byG(f)∝exp(−π2f2/a2)  (2)
For a=100 ps, which corresponds to a pulse with a width of 100 ps, the frequency range can extend to f=5 GHz. Although microwave imaging in the time domain has been studied, typically only back-scattered signals have been utilized, in an approach to detect tumors in breast tissue.